1. Field of the Invention
The present invention relates to an infrared-sensitive planographic printing plate precursor. More particularly, the present invention relates to an infrared-sensitive planographic printing plate precursor with improved prevention of damage to the photosensitive layer when the infrared-sensitive planographic printing plate precursors are stacked.
2. Description of the Related Art
Laser technology has made remarkable progress in recent years. In particular, high-power and compact solid lasers, semiconductor lasers and the like, having an emission wavelength within the near infrared and infrared regions are now readily available. In a planographic printing field, such lasers are advantageously used as light sources for exposing planographic printing precursors so as to produce printing plates directly according to digital data from a computer or the like.
A recording layer of such a positive planographic printing plate precursor for direct plate-making using infrared laser includes as essential components an alkali-soluble resin and an infrared absorbent which absorbs light and generates heat. In unexposed portions (i.e., an image area), the infrared absorbent acts as a dissolution inhibitor, which interacts with the alkali-soluble resin to substantially lower the solubility of the alkali-soluble resin. On the other hand, in exposed portions (i.e., a non-image area), the interaction of the infrared absorbent and the alkali-soluble resin becomes weak due to the heat generated, and the infrared absorbent dissolves in the alkaline developer to form an image. Such a positive planographic printing plate precursor, however, has problems in that the mechanical strength of the recording layer is insufficient. During manufacture, transportation and handling of the printing plate precursor, if the printing surface contacts other components heavily, defects in the printing surface can be generated, and missing portions can appear in the developed image area.
To reduce such problems, planographic printing plate precursors are usually packaged with interleaving sheets interposed between adjacent printing plate precursors. The interleaf sheets, however, have problems of cost and removal. Accordingly, “interleaf sheet-less” is desirable. Recently, as Computer-to-plate (CTP) systems become common, more and more exposure devices are provided with printing plate autoloaders. Such autoloaders, however, have the problem that the interleaf sheets need to be removed in advance from the stack through a bothersome manual operation, and that, even in an autoloader equipped with a device for automatically removing interleaf sheets, the printing plate precursors sometimes become scratched when removing the interleaf sheets. To avoid these problems, demand for planographic printing plate precursors stacked without interleaf sheets is increasing.
A known technique towards packaging without interleaf sheets is to provide supports with a back surface designed to reduce mechanical damage to photosensitive layers caused by contact between the photosensitive layers and the back surface of the supports.
For example, a recording material for offset printing which includes a radiation-sensitive layer and an organic polymer-containing backcoat layer is known. The recording material is provided with a backcoat consisting of an organic polymer having a glass transition temperature of not lower than 35° C., with a pigment such as silica gel contained therein (see Japanese Patent Application Laid-Open (JP-A) No. 2002-46363). The patent document describes that, with this configuration, planographic printing plate precursors can be stacked with no interleaf sheets interposed. However, inorganic pigments such as silica gel are very hard and if contained in the backcoat layer, the pigments can easily cause scratching of photosensitive layers during transportation of the printing plate precursors in a stacked state without interleaf sheets.
Another proposed technique is to provide a matte surface by electrostatic spraying onto a surface on the opposite side to that of a photosensitive layer on a support (see JP-A No. 2003-63162). The patent document describes that, with this configuration, planographic printing plate precursors can be stacked with no interleaf sheets interposed. However, if the printing plate precursors are stacked without interleaf sheets and stored, the adhesion of adjacent printing plate precursors can occur, especially under conditions of high-humidity such as in the summer.
Further, a photosensitive planographic printing plate precursor with a coating layer provided at a surface on the opposite side to that of a photosensitive layer on a support is proposed. The coating layer has a glass transition temperature of 60° C. or above and is formed by at least one resin selected from a group consisting of saturated copolyester resin, phenoxy resin, polyvinylacetal resin and vinylidene chloride copolymer resin (see JP-A No. 2005-62456).
Also, a photosensitive planographic printing plate precursor with a rough-surfaced organic polymer layer provided at a surface on the opposite side to that of a photosensitive layer on a support is also proposed (see, for example, JP-A No. 2002-254843).
These backcoat layers of organic polymer, however, can easily cause scratching of a photosensitive layer if no interleaf sheets are used and the photosensitive layer and the back surface are pressed against each other in an autoloader for automatically feeding printing plate precursors for laser exposure. Further, as mentioned above, a planographic printing plate precursor which has a recording layer including an alkali-soluble resin and an infrared absorbent and a backcoat layer of organic polymer has the following problems. That is, during manufacture after coating and drying printing plate precursors are cut and stacked and when the stacked printing plate precursors get load applied when on an autoloader, the relatively low strength surfaces of recording layers can adhere to the surfaces of backcoat layers due to solvent contained in the backcoat layer, and the recording layers sometimes peel off as a result of this adhesion.